Corp pilot climbs above assigned altitude. Misinterpretation of clearance.

Attributes
ACN	144651
Time
Date	199005
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : dcu
State Reference	AL
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : hsv tower : hpn
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : cfi pilot : instrument pilot : commercial
Experience	flight time last 90 days : 100 flight time total : 3315 flight time type : 575
ASRS Report	144651
Person 2
Affiliation	government : faa
Function	controller : departure
Qualification	controller : radar
Events
Anomaly	altitude deviation : overshoot inflight encounter other other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While en route from bna to pfn on an IFR flight plan in an small transport at 9000', I encountered moderate to heavy mixed icing. I made a request to huntsville approach control to climb to 11000', which looked like it would put me on top of the clouds with clear conditions above. The controller replied by telling me that he had my request and a short while later I heard him say, 'climb and maintain 11000', but I'll have to get clearance if you want any higher.' I immediately started a climb at vy to 11000'. Somewhere around 10400' the controller asked me to verify altitude. I replied, and by then was at 10800'. The controller then stated that he cleared me to 10000' and told me that he would have to get clearance for me to go to 11000'. He instructed me to descend to 10000'. I did so and shortly thereafter was cleared to 11000'. In my mind, I heard the clearance to 11000'. Cockpit noise levels were greatly increased due to ice shedding of the propellers as well as from the forming of the ice. The workload in the cockpit was also at a peak due to the constant monitoring of the ic build-up and cycling of the de-icer boots which was required every 1-1 1/2 mins. Perhaps it was in the controller's explanation that I heard 11000' and that may be when I set my altitude pointer (mounted on altimeter) to 11000'. I'm sure the urgency of the situation had something to do with it, but I was sure that I was cleared to 11000'.

Original NASA ASRS Text

Title: CORP PLT CLIMBS ABOVE ASSIGNED ALT. MISINTERPRETATION OF CLRNC.

Narrative: WHILE ENRTE FROM BNA TO PFN ON AN IFR FLT PLAN IN AN SMT AT 9000', I ENCOUNTERED MODERATE TO HEAVY MIXED ICING. I MADE A REQUEST TO HUNTSVILLE APCH CTL TO CLB TO 11000', WHICH LOOKED LIKE IT WOULD PUT ME ON TOP OF THE CLOUDS WITH CLR CONDITIONS ABOVE. THE CTLR REPLIED BY TELLING ME THAT HE HAD MY REQUEST AND A SHORT WHILE LATER I HEARD HIM SAY, 'CLB AND MAINTAIN 11000', BUT I'LL HAVE TO GET CLRNC IF YOU WANT ANY HIGHER.' I IMMEDIATELY STARTED A CLB AT VY TO 11000'. SOMEWHERE AROUND 10400' THE CTLR ASKED ME TO VERIFY ALT. I REPLIED, AND BY THEN WAS AT 10800'. THE CTLR THEN STATED THAT HE CLRED ME TO 10000' AND TOLD ME THAT HE WOULD HAVE TO GET CLRNC FOR ME TO GO TO 11000'. HE INSTRUCTED ME TO DSND TO 10000'. I DID SO AND SHORTLY THEREAFTER WAS CLRED TO 11000'. IN MY MIND, I HEARD THE CLRNC TO 11000'. COCKPIT NOISE LEVELS WERE GREATLY INCREASED DUE TO ICE SHEDDING OF THE PROPS AS WELL AS FROM THE FORMING OF THE ICE. THE WORKLOAD IN THE COCKPIT WAS ALSO AT A PEAK DUE TO THE CONSTANT MONITORING OF THE IC BUILD-UP AND CYCLING OF THE DE-ICER BOOTS WHICH WAS REQUIRED EVERY 1-1 1/2 MINS. PERHAPS IT WAS IN THE CTLR'S EXPLANATION THAT I HEARD 11000' AND THAT MAY BE WHEN I SET MY ALT POINTER (MOUNTED ON ALTIMETER) TO 11000'. I'M SURE THE URGENCY OF THE SITUATION HAD SOMETHING TO DO WITH IT, BUT I WAS SURE THAT I WAS CLRED TO 11000'.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.